Astronomy & Astrophysics manuscript no. AB_Aur_final c ESO 2015 May 12, 2015 Chemical composition of the circumstellar disk around AB Aurigae S. Pacheco-Vázquez1 , A. Fuente1, M. Agúndez2, C. Pinte6, 7, T. Alonso-Albi1, R. Neri3, J. Cernicharo2,J. R. Goicoechea2, O. Berné4, 5, L. Wiesenfeld6, R. Bachiller1, and B. Lefloch6 1 Observatorio Astronómico Nacional (OAN), Apdo 112, E-28803 Alcalá de Henares, Madrid, Spain e-mail: [email protected], [email protected] 2 Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, C/ Sor Juana Inés de la Cruz 3, E-28049 Cantoblanco, Spain e-mail: [email protected] 3 Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d’Hères, France 4 Université de Toulouse, UPS-OMP, IRAP, Toulouse, France 5 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France 6 Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Université UJF-Grenoble 1/CNRS-INSU, F-38041 Grenoble, France 7 UMI-FCA, CNRS/INSU, France (UMI 3386), and Dept. de Astronomía, Universidad de Chile, Santiago, Chile e-mail: [email protected] Received September 15, 1996; accepted March 16, 1997 ABSTRACT Aims. Our goal is to determine the molecular composition of the circumstellar disk around AB Aurigae (hereafter, AB Aur). AB Aur is a prototypical Herbig Ae star and the understanding of its disk chemistry is paramount for understanding the chemical evolution of the gas in warm disks. Methods. We used the IRAM 30-m telescope to perform a sensitive search for molecular lines in AB Aur as part of the IRAM Large program ASAI (A Chemical Survey of Sun-like Star-forming Regions). These data were complemented with interferometric observa- tions of the HCO+ 1→0 and C17O 1→0 lines using the IRAM Plateau de Bure Interferometer (PdBI). Single-dish and interferometric data were used to constrain chemical models. + Results. Throughout the survey, several lines of CO and its isotopologues, HCO , H2CO, HCN, CN, and CS, were detected. In addition, we detected the SO 54→33 and 56→45 lines, confirming the previously tentative detection. Compared to other T Tauri and Herbig Ae disks, AB Aur presents low HCN 3→2/HCO+ 3→2 and CN 2→1/HCN 3→2 line intensity ratios, similar to other transition disks. AB Aur is the only protoplanetary disk detected in SO thus far, and its detection is consistent with interpretation of this disk being younger than those associated with T Tauri stars. Conclusions. We modeled the line profiles using a chemical model and a radiative transfer 3D code. Our model assumes a flared disk in hydrostatic equilibrium. The best agreement with observations was obtained for a disk with a mass of 0.01 M⊙,Rin =110 AU, ◦ Rout =550 AU, a surface density radial index of 1.5, and an inclination of 27 . The intensities and line profiles were reproduced within a factor of ∼2 for most lines. This agreement is reasonable considering the simplicity of our model that neglects any structure within the disk. However, the HCN 3→2 and CN 2→1 line intensities were predicted to be more intense by a factor of >10. We discuss several scenarios to explain this discrepancy. Key words. stars: formation – stars: individual: AB Aur – stars: pre-main sequence – stars: variables: T Tauri, Herbig Ae/Be – circumstellar matter – protoplanetary disks 1. Introduction very few molecular detections. This scarcity of molecules seems arXiv:1503.04112v2 [astro-ph.EP] 11 May 2015 more accentuated in disks around Herbig Ae stars (Öberg et al. Circumstellar disks are commonly observed around pre-main se- 2011). This is mainly due to the low molecular abundances in quence stars (e.g., Howard et al. 2013; Strom et al. 1989). The a gas disk that itself has a low mass content. The ultraviolet ra- formation of disks, together with ejecta phenomena such as out- diation from the central star photodissociates molecules in the flows and jets, dissipate away the excess of angular momentum surface layers of the disk. Deeper in the midplane, the tempera- that prevents accretion from the parent cloud. The chemical com- tures drop, and all the detectable molecules freeze out onto dust position of dust and gas contained in these disks provides infor- grains. As a result, molecules can only survive in the gas phase mation about the initial conditions in the formation of planetary inside a thin layer. For F and A stars with effective temperatures systems (Dutrey et al. 2014). in the range between 6000 to 10000 K, the UV-photons pene- trate deeper into the disk than the colder M and K stars (T The comprehensionof chemistry in disks is an importantstep eff ∼ toward understanding of the formation of complex organic, even 2500 - 5000K), causing the drop of the molecular detection rates. Most species detected are simple molecules, molecular prebiotic molecules on planets. However, the disk chemistryisa radicals, and ions, such as CO, 13CO, C18O, CN, CS, C34S, C H, very unexplored field from the observational point of view with 2 Article number, page 1 of 15 A&A proofs: manuscript no. AB_Aur_final 13 + 13 + + Table 1. Observed band ranges with IRAM 30-m telescope. HCN, H CN, HNC, DCN, HC3N, HCO , H CO , DCO , + + H2D ,N2H , c-C3H2,H2CO, H2O, and HD (e.g., Kastner et al. 1997; van Dishoeck et al. 2003; Thi et al. 2004; Qi et al. 2008; Band Frequency rms ∆v HPBW ηb −1 ′′ Guilloteau et al. 2006; Piétu et al. 2007; Dutrey et al. 2007). [GHz] (mK) (kms ) ( ) E090 84.5 - 96.3 5-7 0.6 29-25 0.85 Unfortunately, most disks remain unresolved even with the E150 133.8- 144.8 5-8 0.4 18-17 0.79 largest millimeter interferometers. A detailed study of the chem- E230 200.5- 272.0 5-16 0.3 12-9 0.67-0.6 ical composition of the gas in a disk requires not only high an- gular resolution observations in dust continuum and molecular lines, but also accurate chemical and physical models. These 2. Observations models help to constrain the disk structure in accordance with 2.1. IRAM 30-m radiotelescope the observations and calculate the molecular abundance pro- files. In recent years, the chemical and radiative transfer mod- We carried out a spectral survey toward AB Aur ◦ els have improved their performances, (see, e.g., Thi et al. 2013; (αJ2000=04h55m 45.8s, δJ2000=30 33’ 04”.2) as part of Pinte etal. 2010; Nomuraetal. 2009; Agúndezetal. 2008; the IRAM Large program ASAI using the IRAM 30-m tele- Dutrey et al. 2007). scope at Pico Veleta (Granada, Spain). Several observing periods were scheduled on July and February 2013 and January and Herbig Ae/Be stars are intermediate-mass pre-main se- Mars 2014, to cover the 1, 2, and 3 mm bands shown in Table 1. quence sources that emit much stronger thermal UV radiation The Eight MIxer Receivers (EMIR) and the fast Fourier Trans- than do T Tauri stars (TTs). Therefore, their circumstellar disks form Spectrometers (FTS) with a spectral resolution of 200kHz are warmer and more ionized. Our target, AB Aurigae (here- were used for the whole project. The observing procedure was after, AB Aur) is one of the best-studied Herbig Ae stars that wobbler switching with a throw of 200′′ to ensure flat baselines host a prototypical Herbig Ae disk. It has a spectral type A0-A1 and to avoid possible contamination from the envelope toward (Hernández et al. 2004). It has a M⋆∼ 2.4 M⊙ ,aTeff ∼ 9500 K, this young disk. In Table 1 we show the beam efficiency, half and it is located at a distance of 145 pc (van den Ancker et al. power beam width (HPBW), spectral resolution, and sensitivity 1998). The disk around AB Aur shows a complex structure. It achieved in each observed frequency band. ′′ is larger (Rout∼1100 AU, ≈7 ), than those around TTs, and it The data reduction and the line identification were carried shows spiral-arm features traced by millimeter continuum emis- out with the package CLASS of GILDAS software (Maret et al. sion, at about 140 AU from the star (Piétu et al. 2005). 2011). Three databases were used to identify the lines: (1) Cologne Database for Molecular Spectroscopy (CDMS; see By modeling the 12CO and its isotopologue lines obtained Müller et al. 2005), (2) the molecular spectroscopy database of from subarcsec imaging with the Plateau de Bure Interferome- Jet Propulsion Laboratory (JPL; see Pickett et al. 1998), and (3) ter (PdBI), it was found that, contrary to typical disks associ- MADEX (Cernicharo 2012). ated with TTs, the AB Aur disk is warm (>25 K all across the disk) and shows no evidence of CO depletion (Piétu et al. 2005). Schreyer et al. (2008) carried out a chemical study of the disk 2.2. Plateau de Bure interferometer + → around AB Aur with the PdBI searching for the HCO 1 0, CS The interferometric observations were carried out in the sec- → → → 2 1, HCN 1 0, and C2H1 0 lines, but with only one detec- ond half of 2011 using the C and D configurations of + → tion, the HCO 1 0 line. They propose that the poor molec- the PdBI with six antennas. This configuration provided a ular content of this disk is because the UV-photons dissociate beam of ≈5.41′′×4.31′′ with a position angle (PA) of 129◦ molecules. More recently, Fuente et al. (2010) have carried out at the frequency of HCO+ 1→0 (89.1885 GHz) and of a molecular search using the IRAM 30-m telescope.